Focus: If I Could Turn Back Time

The furniture in a cluttered room allows two wireless signals to be transmitted simultaneously to different locations using the same frequency.

Buildings and trees are not always a nuisance for radio communications. They could actually improve transmission to a cell phone or radar station, according to research in the 10 December PRL. A team used furniture clutter to simultaneously focus two different electromagnetic signals to two specific locations in a room using a single frequency, with an accuracy that would be impossible in an empty room. The trick was to first send signals in the reverse direction, from the target positions, and measure the distortions caused by the clutter. The demonstration suggests that previously proposed techniques for improving radio-frequency communications can work in the real world.

Suppose you want to make ripples in a puddle that form concentric circles moving inward toward a specific spot, but your only tools are a set of pebbles. And suppose that the puddle is strewn with twigs and rocks. The solution is to first drop a pebble at the spot in the middle and then watch carefully as the waves radiate outward, scatter and break up as they hit the obstacles, and ultimately hit the edges in some complicated pattern. Now drop the pebbles at the edges in a way carefully timed to create the time-reversed version of the complicated pattern. Somewhat like running a movie in reverse, the waves will then flow toward the center and add up to make inward-radiating circles at the original impact point.

Researchers have used an equivalent technique–with a set of sound transmitters in place of the pebbles–to focus sound waves to specific points in a water tank full of steel rods [1]. Obstacles improve focusing because they provide a wide variety of routes through which the original “forward” signal can travel. The different path lengths lead to different signal distortions and delay times for each transmitter to imitate when it sends the reversed signal. In the obstacle-free case, each transmitter receives a similar signal, so the waves don’t contain as much information on the target location. Although the same team has also time-reversed electromagnetic waves in a highly-reflective box [2] until now no one has demonstrated the spatial focusing effect with electromagnetic waves in a normal room–the real environment for wireless communications.

Benjamin Henty and Daniel Stancil of Carnegie Mellon University in Pittsburgh used an array of four transmitter antennas located near one another but 7 meters from the target receiver at the other end of a desk- and computer-cluttered office. Using 2.45 gigahertz radio waves, they first demonstrated that the array could focus the reversed radio signal down to a region around the target just 5 centimeters wide. Without the furniture, the signal “hot spot” spread over about a meter, according to readings from the receiver placed at many locations. Next the team used two targets, 5 centimeters apart. After recording signals sent from each, they added the reversed signals together and simultaneously focused separate transmissions to each location using the same frequency.

The researchers believe their technique could be used to enhance wireless communications or radar. Cell phones could send “pilot waves,” or steady tones to receiving stations, says Stancil, which would then send multiple signals more precisely to cell phones on the same frequency. In radar the method could enhance the signal from a detected object.

“This is very promising for radio frequency applications in cities and indoors,” says Geoffroy Lerosey of the University of Paris, part of the group that first time-reversed electromagnetic signals. Time reversal could also help reduce signal degradation problems that could face future wide-band wireless networks that would carry far more information than today’s technology, he says.

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Phys. Rev. Lett. 93, 243904 (2004)

Clutter is good. The disordered furniture in this room allowed two different radio transmissions to be beamed simultaneously to nearby locations using a single frequency. Cell phones and other wireless communications could benefit from such techniques.